#include<iostream>
#include<cstdio>
#include<cctype>
#include<vector>
#include<queue>
#include<algorithm>
using namespace std;
const int maxn=1100010;
int n,p,c,m,u,v,d[maxn],ans;
vector<int> g[maxn];
bool inq[maxn];
queue<int> q;
inline void in(int &x){
    x=0;int f=1;char t=getchar();
    while(!isdigit(t)){if(t=='-')f=-1;t=getchar();}
    while(isdigit(t)){x=x*10+t-48;t=getchar();}
    x*=f;
}
inline void spfa(int c){
	q.push(c);inq[c]=1;d[c]=1;
	while(!q.empty()){
		int now=q.front();q.pop();
		for(int i=0;i<g[now].size();i++){
			if(!inq[g[now][i]]){
				d[g[now][i]]=d[now]+1;
				q.push(g[now][i]);
				inq[g[now][i]]=1;
			}
		}
	}
}
inline void work(){
	in(n);in(p);in(c);in(m);
	for(int i=1;i<=p;i++){
		in(u);in(v);
		g[u].push_back(v);
		g[v].push_back(u);
	}
	spfa(c);
	ans=*max_element(&d[1],&d[n]);
	printf("%d",m+ans);
}
inline int mian(){
	freopen("dividecandy.in","r",stdin);
	freopen("dividecandy.out","w",stdout);
	work();
}
int miku=mian();
int main(){;}